A single phase induction motor comprises essentially a wound annular laminated stator, which is energized by a single phase a.c. supply, and a shorted rotor of the cage type in which voltages are induced by the variation of primary voltage in the stator winding. The application of single phase a.c. to the stator run winding creates an oscillating magnetic field which provides a torque to drive the motor once it is rotating. However, the oscillating field alone will not start the motor. In order to start this type of motor an auxiliary or start winding is provided which is spatially displaced around the stator from the run winding. The supply voltage is applied to both windings but is time phase shifted in the start winding. This phase shift causes a rotating rather than an oscillating field to be produced, which is sufficient to cause the rotor to turn.
Many ways of effecting this phase shift are known, the simplest being to make the run and start windings of different inductance. This is the so-called "split-phase" motor. Another equally common technique is to employ a phase shifting capacitor which allows a more efficient 90.degree. phase shift to be introduced. The start winding is switched out of circuit when the motor is rotating so that the motor runs solely on the single phase a.c. supplied to the run winding. This aspect of single phase motor operation is very well known and is discussed in various books on the subject. One such book, for example is, "Electric Motors Handbook", McGraw-Hill Book Company, 1978 in which Chapter 7 is of particular interest.
A single phase supply motor which is not only started but also run as a two-phase motor by a somewhat different method is shown in UK Pat. No. 1,464,454. As with conventional single phase motors, two spatially displaced windings are provided on the stator one of which is supplied directly with a.c. The other winding is centre tapped and each half of this winding may be supplied with rectified mains (power line) voltage by way of a respective thyristor. A gating circuit switches the thyristors alternately at predetermined points in the mains cycle so that the second winding is energized with an alternating, though not sinusoidal, voltage which is phase shifted from the power line (mains) by a fixed amount. Because of the phase shift a rotating field is produced which starts the motor. In this case the two phase arrangement is also used to run the motor as well.
In many applications, the a.c. supply to the run winding of a single phase induction motor is not provided directly from the power line (mains) but is provided electronically from, say, an inverter. UK Pat. No. 1,347,191 shows such a system and points out that under constant speed and constant load running conditions, the power required to run such a motor is often very much less than that which is needed for starting. This means that the inverter must be designed to provide starting currents very much in excess of those needed during running. The patent proposes the use of the a.c. power line to supply the high currents needed during starting after which the motor is switched to the electronically generated a.c. supply for running. The generated a.c. is synchronized with the power line prior to switchover. In this way the generating circuit (inverter) need only be designed for running conditions.
A similar arrangement to that of UK Pat. No. 1,347,191 is decribed in an article by R C Treseder entitled "Coaxial Drive for Magnetic Disk File" published in the IBM Technical Disclosure Bulletin, Volume 23, No 3, August 1980, page 1198, which shows a four pole three phase induction motor for driving a magnetic disk file. The optimum supply frequency for the operating speed of the disk file is 120 Hz which is provided to the motor by an oscillator driven three phase power amplifier. To provide higher starting torque than would be available from the 120 Hz generated supply, the motor is started by connection to three phase a.c. power line. When the motor has reached its mains synchronous speed, which is sufficient for the heads to fly, the power line is disconnected and the 120 Hz supply switched to drive motor.